Recently, due to a trend in miniaturization of electronic equipment, electronic components for electronic equipment also have become significantly smaller in size, and yet constructed as multifunctional components having a number of functions. Such multifunctional components include BGA, CSP and the like, which is configured to include a number of electrodes disposed therein. When a multifunctional component is to be implemented in a printed board, solder is applied between the electrodes and lands of the printed board.
Other types of electronic component, such as QFP and SOIC, are configured to include a bare chip having internally a number of, electrodes, that are connected to the board of the electronic component by soldering.
In the soldering process as described above, if solder is separately and individually supplied to every one of a number of locations of placement or to significantly small electrodes, an excessive labor must be necessary. In addition, solder cannot be supplied precisely to each one of a respective micro soldering spot. Accordingly, in the practice of soldering involving multifunctional components or a bare chip, an amount of solder is previously attached to the electrode so as to form a solder bump thereon, which is then melted during soldering to produce a soldered connection. Generally, a solder sphere is used for forming a solder bump.
For formation of such solder bump, processes using solder paste, a solder sphere and the like are adopted. Traditionally, a process using solder paste, which is inexpensive in terms of the cost, has been adopted predominantly. However, under recent circumstances where a micro size of formed bump in a range of 30-200 μm is required, or owing to a fact that a height of implementation can be more reliably achieved by a bump formed a solder sphere, a process using a solder sphere having a diameter equal to a required bump height has become common in practice, though it is expensive in terms of the cost. Specifically, use of solder spheres is essential in an electrode for an external terminal of a BGA and CSP or an electrode for a bare chip connection inside a component, where achieving reliably a consistent height in implementation is of great importance.
To amount solder spheres on a number of electrodes, the solder spheres are introduced into a pallet with holes having a diameter smaller than the solder spheres formed therethrough. The pallet is vibrated to thereby seat the solder spheres in the holes in line with each other within the pallet. Then, the solder spheres are mounted on a solder sphere mounting head. Accordingly, if an aspect ratio of a solder sphere is large and/or there is larger deviation in grain diameter, the solder sphere cannot be loaded successfully on the electrode. Thus, it is important to ensure that there is no deviation in grain diameter of every one of the solder spheres in order to achieve reliably a precise amount of solder, and thus a consistent height of implementation.
The solder sphere, i.e. the subject of the present invention, is referred to as the solder in a spherical form used in implementation, and for use in the mounting process as described above, must satisfy conditions, including: (1) having a sphericity of solder sphere not less than 0.95, and a fixed grain diameter with less distortion; (2) having no contamination on the surface of the sphere; (3) having less rougher and smooth surface; (4) having no relatively thick oxide film over the surface; and (5) having a fixed content of alloy composition.
To achieve the foregoing, a container for storing the solder spheres must also be such that will not affect a grain diameter of a solder sphere. Moreover, it is required to prevent, in addition to any deformation due to impact from the outside to the solder spheres, such as the phenomenon referred to as blacking that occurs when the solder spheres move and rub against each other within the container, leading to cracks in the surfaces of the spheres, resulting in solder powders, which oxidize and blacken. In order to prevent such blacking, a known solution has suggested a cylindrical container body having a bottom an opening of which is sealed with a lid having an inwardly protruding member so as to reduce a space available for movement of the solder spheres (Patent Literature 1).
In addition, as the solder spheres become smaller, and thus the ratio of surface area to total volume of the solder spheres increases, the surfaces of the solder spheres are more likely to become oxidized and turn yellow. Such yellowing of the solder spheres is due to the fact that the solder spheres are exposed to the atmosphere and Sn in the solder spheres is oxidized by oxygen in the atmosphere. As the oxide film of the Sn colors yellow, the film, as it becomes thicker, causes the entire solder sphere to appear yellowish.
Mounting of the solder spheres, such as in the BGA implementation, in which the solder spheres are aligned on the pallet and mounted together as a block requires that a presence of the solder spheres be confirmed by an image recognition device, after mounting of the solder spheres. In this process, any yellowish coloring of the solder spheres may cause an error detected in the image recognition device. Such an error, once detected by the image recognition device, may cause a stoppage of the production line, thereby seriously affecting productivity.
In addition, if a surface of the solder sphere is covered with oxide film, such oxide film may on occasion not be broken during melting of the solder sphere, and may thus remain on the electrode as held in the sphere profile or adhere to the electrode, which may inhibit wetting by the melted solder and lead to bad soldering.
In light of the circumstances as noted above, some types of containers directed to prevent oxidization and yellowing of Sn-based lead-free solder spheres have been suggested. (Patent Literature 2 to 5).
A simple but effective method for preventing yellowing of solder spheres is to pack solder spheres in a laminated sheet or an aluminum sheet that is impermeable to air and from which air is evacuated and then sealed with solder spheres loaded therein (Patent Literature 2). It is also possible to include a deoxidant or absorbent or a buffering member enclosed together in the inside thereof.
There is another known method, in which a space for receiving a deoxidant is created inside a solder sphere storing container having an oxygen barrier property as well as conductivity, so that inclusion of the deoxidant received in said space may function to prevent oxidization of the solder spheres (Patent Literature 3).
There are other known methods, including one using, instead of the deoxidant received in the container, a container comprising a resin material that contains an antioxidant component or another using a member containing the antioxidant component, which is received together with the solder spheres inside the container (Patent Literature 4).
In yet another known method, an outer lid of the container body is adhered with a seal in order to prevent oxidization of the solder spheres (Patent Literature 5). According to this method, once the seal is removed and the container is placed in an unsealed condition, the solder spheres inside must all be consumed, as oxygen will flow into the container and the oxidizing process will start after unsealing of the container. Any solder spheres remaining unused will therefore no longer be usable, as they will be oxidized. Accordingly, the bad soldering due to the oxide film may be prevented.
Though not specifically a storage container for solder spheres, there is a known packaging method for storing a metal wiring material, such as a wire and a ribbon, made of metal, such as copper and solder, that is more likely to be oxidized (Patent Literature 6). According to this method, the metal wiring material is wound around a spool, which is contained in a plastic case, and the whole case along with a deoxidant is sealed by a laminated sheet.
Citation List
Patent Literature
PTL 1: Japanese Patent Laid-open Publication No. 2000-335633
PTL 2: Japanese Patent Laid-open Publication No. 2003-312744
PTL 3: Japanese Patent Laid-open Publication No. Hei11-105940
PTL 4: Japanese Patent Laid-open Publication No. 2007-230613
PTL 5: Japanese Patent Laid-open Publication No. 2008-37487
PTL 6: Japanese Patent Laid-open Publication No. Hei03-289415